Because of their high energy density and output power, lithium secondary batteries have received great attention as candidates for making high performance batteries which can be used in a wide spectrum of applications such as notebook PC's, mobile phones, and many other portable devices. Polymer-electrolyte-based lithium secondary batteries have attracted particular attention because of their perceived huge potential in consumer and other electronic goods. In addition to the desirable characteristics typically provided by lithium secondary batteries, polymer electrolyte batteries, which typically exist in the form of a film-type solid state battery, further provide the advantages of relative ease of manufacturing and excellent shapability. These advantageous properties meet many of the demands of today's information industry, and have generated significant research and development interests in attempt to further improve the properties of polymer electrolyte for use in lithium secondary batteries.
In a paper presented at the Second International Meeting on Solid Electrolytes held at St. Andrews, Scotland (Sep. 20-22, 1978), M. B. Armand, J. M. Chabagno and M. Duclot disclosed that a complex of poly(ethylene oxide), or PEO, and lithium ions can be used as a solid electrolyte in lithium batteries. However, after repeated research and development efforts, it was found that the conductivity of PEO-based polymer electrolytes at temperatures between 40 and 100.degree. C. can only reach about 10.sup.-8 to 10.sup.-4 S-cm.sup.-1. Such a low ionic conductivity severely limits the potential use of PEO-based electrolytes in lithium secondary batteries.
In 1993, in an article entitled "Li ion conductive electrolytes based on poly(vinyl chloride)", J. Electrochem. Soc., 140, L96-97, M. Alamgir and K. M. Abraham disclosed a PVC (poly(vinyl chloride))-based polymer electrolyte which used THF (tetrahydrofuran) as a carrier solvent. The lithium ion conductivity of their PVC cells was improved to 10.sup.-3 S-cm.sup.-1.
The use of PVC-based polymer electrolyte had been disclosed by M. Watanabe, et al, who, in an article entitled "High lithium ionic-conductivity of polymeric solid electrolytes," Makromol. Chem. Rapid. Comm. 2, pp 741-744 (1981), taught a solid polymer electrolyte composed of solvates of Li salts immobilized in a solid organic polymer matrix containing PVC dissolved in a mixture solvent of EC (ethylene carbonate) and PC (propylene carbonate). However, because neither EC nor PC is a good solvent for PVC, the resultant polymer electrolyte exhibited unsatisfactorily poor lithium ion conductivity. The use of THF as a carrier solvent, as disclosed by Alamgir et al, ameliorated the solubility problem. However, this improvement was found not to endure. With time, the carrier solvent THF gradually evaporated, and, as a result, it was found that the EC/PC molecules would exude from the polymer matrix. When this occurred, it was also observed that the THF-modified-PVC-based polymer electrolyte disclosed by Alamgir et al experienced a precipitous drop in its lithium ion conductivity, from the high of 10.sup.-3 S-cm.sup.-1 to below 10.sup.-7 S-cm.sup.-1.
Because of the significant potential market of solid state polymer electrolytes, it is highly desirable to develop new and improved materials that will overcome the shortcomings observed in the prior art materials.